The invention relates to a process for the production of polyamide-6 from xcex5-caprolactam, comprising
(a) the polymerization of xcex5-caprolactam to polyamide-6 at a temperature above the melting point of polyamide-6,
(b) recovery of polymerizate in solid form from polymerization step (a),
(c) extraction of the solid polymerizate obtained in (b), using a suitable solvent, with removal of non-converted monomer and oligomeric compounds from the polymerizate,
(d) separation of the extracted polymerizate from the extraction agent,
(e) concentration (by evaporation of the solvent) of the solution of non-converted monomer and oligomeric compounds which has resulted from the extraction and
(f) recycling of the concentrate obtained in (e) as starting material to the polymerization (a).
Such a process is disclosed in U.S. Pat. No. 4,053,457 of BASF and in Fibre Producer, June 1981, pp. 20 and 22 by G. Panks c.s. In said process the polymerization as a whole is carried out in a polymerization column (a so-called VK column), the polymerizate is washed with water, the resulting extract, which mainly consists of xcex5-caprolactam and a smaller fraction of oligomeric compounds, is concentrated by evaporation to a low water content and subsequently supplied, preferably as a melt, to the top of the VK column together with fresh caprolactam as starting material for the polymerization.
However, this process suffers from the drawback that cyclic dimer (CD) is formed and accumulated. If this CD is present in the polyamide-6, it may give rise to problems in the processing of the polyamide to for instance film and fibre owing to its relatively high melting point, 347xc2x0 C. As a remedy for this, U.S. Pat. No. 5,218,080 of BASF, discloses a processxe2x80x94which was also already described in U.S. Pat. No. 4,053,457xe2x80x94wherein the concentrated extract to be recycled is subjected to a thermal treatment at a temperature of between 200 and 300xc2x0 C. and at a pressure in excess of the vapour pressure of the concentrate. In the examples presented in said patent publications such a treatment is carried out in an autoclave for a relatively long period of 4 hours in the presence of about 20 wt. % of water. This process results in a reduction of the cyclic dimer content of the concentrate to less than 1.3 wt. % of CD, so that, if the concentrate is supplied to the polymerization column together with fresh xcex5-caprolactam, the CD content during the polymerization remains considerably below the equilibrium concentration of the CD in the reaction mixture. The equilibrium concentration of the CD depends, among other factors, on temperature, pressure and water content during the polymerization and under the customary polymerization conditions it lies between 0.8 and 1.0 wt. % relative to the amount of initial monomer.
In other known process embodiments the extracted monomer and oligomers are not recycled any more to the polymerization process, but subjected to a hydrolysis in a separate process line in order to convert i.e. cyclic dimer fully to xcex5-caprolactam. The latter process offers the advantage that the CD content of the polymerizate leaving the polymerization reactor can be kept at the lowest possible level and the polyamide-6 obtained after extraction is able to meet very strict specifications in respect of CD content, which is of importance in particular in the case of highly failure-prone applications of polyamide-6, such as fibre spinning and film drawing. The latter state-of-the-art process and other processes mentioned before in this application suffer from the drawback that a separate treatment of the extracted non-converted xcex5-caprolactam and CD is required, involving additional investments and expenditure.
The object of the invention under the present application is a process for the preparation of polyamide-6 in which at least full re-use of the non-converted monomer can take place without involving the necessity of the additional investment for a hydrolysis treatment of the recyclate.
A further object of the invention is a process for the preparation of polyamide-6 from xcex5-caprolactam which allows a highly fluctuating share of recycled xcex5-caprolactam concentrate with a fluctuating proportion of cyclic dimer in the xcex5-caprolactam feed.
These objectives of the invention are realized by carrying out the polymerization in at least two successive steps, namely
(a1) at a temperature between 220 and 285xc2x0 C. and at an elevated pressure which remains below the vapour pressure of the reaction mixture supplied, with solvent being discharged via the vapour phase, for such a time that the polymerization process is initiated and
(a2) while letting down the pressure prevailing in (a1) to approximately atmospheric or, if desirable, reduced pressure, the reaction mixture obtained in (a1), further called xe2x80x98the prepolymerizatexe2x80x99, is polymerized further, at a temperature above the melting point of polyamide-6, until the required degree of polymerization is reached.
Next, the polymerizate is recovered in the customary manner from the polycondensation reactor, which will in general be a conventional so-called VK column. The polymerizate is then subjected to a washing (extraction) treatment with a suitable solvent, for instance water or methanol, optionally at elevated temperature. This involves removal of low-molecular extractable residues, mainly non-converted xcex5-caprolactam and various oligomeric compounds, from the polyamide-6.
The extract, which preferably has the highest possible content of extracted material, for instance 10 wt. %, is subsequently concentrated by evaporation and recycled as feed, optionally together with the extract from one or more other polyamide-6 production lines, to the prepolymerization step (a1). In the most extreme case the feed for the polymerization can consist completely of extract from several production lines.
The polymerization process of xcex5-caprolactam to polyamide-6 with a prepolymerization step (a) and a condensation step (b) is known per se. The current modes of embodiment of this can also be employed in the process according to the invention.
The process according to the invention is highly suitable for a continuous mode of embodiment.
Optionally, in step (a1) a catalyst is used. The commonly employed catalysts for the production of polyamide-6 from xcex5-caprolactam are suitable as prepolymerization catalyst, for instance allyl lactam, polyamide-6,6 salt and organic acids such as acetic acid, propionic acid and benzoic acid, phosphoric acid and phosphate salts.
Where in the present application the term xe2x80x98polyamide-6xe2x80x99 is used, it is understood to mean the homopolyamide and the copolyamides of which at least 50% of the chain units are derived from xcex5-caprolactam.
The concentrate is preferably recycled completely to the prepolymerization step (a1), because in this way the most stable process operation is ensured. In general fresh lactam is additionally supplied to the prepolymerization step. Depending on the requirements to be met by the polymerization process as a whole, the total amount of fresh lactam can already be present in prepolymerization step (a1). Especially if a large fraction of the lactam feed consists of recyclate it is recommendable not to introduce at least part of the fresh lactam until the polycondensation step (a2).
The process according to the invention offers special advantages where at one production site several production lines with optionally different production capacities are present for the production of polyamide-6 from caprolactam. This is especially the case if the product of one or more production lines has to meet very high requirements in respect of purity and reproducibility. In such a situation recycling of extracted lactam and CD can be omitted on one or optionally more production lines without loss of feedstock, while one other production line is employed for complete re-use according to the invention of the extracted lactam from these production lines. This can be realized for instance by separately concentrating by evaporation and then collecting the extract from the various production lines. It is preferable, however, to collect the extract flows from the various production lines and to concentrate these in one evaporation unit and then use them in the process according to the invention.
In the case of the state-of-the-art processes, with only a VK column and without a special pretreatment, the above embodiments would result in unacceptably high CD content values and a highly failure-prone process. If the process according to the invention is used, being on-stream or off-stream of one of the production lines the extract from which is collected will only have a minimal effect on the composition of the polymerizate obtained from (a2).
The process according to the invention offers much flexibility. The hold-up time in the prepolymerization step can vary within wide limits, for instance between 0.5 and 8 hours, and is preferably chosen between 1 and 6 hours, while both shorter and longer hold-up times may offer an additional advantage in specific cases. The hold-up time in the condensation reactor can also vary within wide limits, for instance between 2 and 12 hours, while both shorter and longer hold-up times may offer an additional advantage in specific cases. For reasons of business economics the aim will in general be to have such a combination of hold-up times in steps (a1) and (a2) that the overall hold-up time (a1)+(a2) is as short as possible. The overall hold-up time across (a1)+(a2) is preferably less than 14 hours. This mainly depends on the desired polymerization degree of the polyamide-6 and the conditions of the polycondensation step and the amount of chain terminator applied.
The polymerizate leaving the polycondensation reactor (a2) generally has a CD content which is about equal to the equilibrium concentration of 0.8-1.0 wt. %. This means that the conditions for the extraction step (d) can be defined well. In contrast to the current views that the lowest possible CD content of the polymerizate should be aimed at, it appears that the CD content in the polyamide-6 in the process according to the invention can be reduced to a low level, i.e.  less than 0.1 wt. %, with customary extraction techniques for the lactam washing, resulting in a polyamide-6 which is widely employable, for instance for engineering plastic applications.
The invention will now be further elucidated by means of the following examples and comparative experiments.